SUB- KINGDOM III.—ANNULOIDA. 



CHAPTER X. 



ECHINODERMATA. 



The third primary division of the animal kingdom is known 

 by the name of Annuloida, and includes two groups of organ- 

 isms which are extremely unlike one another in appearance, 

 and are termed respectively the JEchinodermata and the Sco- 

 lecida. In the former we have the sea-urchins, star-fishes, and 

 their aUies, formerly classed in the old sub-kingdom Hadiata ; 

 in the latter are a number of internal parasites, with some 

 minute aquatic creatures, all formerly referred elsewhere. Dif- 

 ferent as are these two groups in appearance and habits, they 

 are nevertheless united by the following peculiarities : * They 

 possess a distinct alimentary canal, usually communicating 

 with the outer world by two apertures {a mouth and a vent), but 

 in any case completely shut off from the general cavity of the 

 body. In all there is a distinct nervous system ; and in all 

 there is a peculiar system of canals termed the ^''water-vascu- 

 lar " or " aquiferous " vessels, which usually communicate 

 with the exterior of the body. It should be mentioned that 

 many naturalists dissent from this grouping together of the 

 Echinodermata and Scolecida into a single sub-kingdom, 

 Annuloida. Many other arrangements have been proposed, 

 most of which present some special advantages and-some dis- 

 advantages. In the mean while, in the confessedly uncertain 

 state of this department of Natural History, it has been thought 

 well to adhere to the arrangement proposed by Prof. Huxley, 

 an arrangement with many obvious drawbacks, and at best but 

 provisional. 



* Some of the internal parasites of this sub-kinffdom have no alinaentary canal at all , 

 but this does not affect the value of the above definition. 



